(1) Technical Field
This invention relates to electronic circuitry, and more particularly to radio frequency circuits.
(2) Background
In electronics, a frequency mixer (or just “mixer”) is an electrical circuit that creates new frequencies from two applied signals. More particularly, in their simplest form, mixers are 3-port active or passive devices designed to yield both a sum and a difference frequency at a single output port when two distinct input frequencies are inserted into the other two ports. Thus, when two signals at frequencies f1 and f2 are applied to a mixer, the intended output is a product signal of the sum f1+f2 and difference f1−f2 of the original frequencies. In addition, the inputs to a mixer may be complex waveforms, such as a signal spectrum (e.g., in CDMA cellular telephone systems) or a broad spectrum of signals (e.g., cable TV). In practice, mixers also produce harmonic frequencies and higher order sum and difference frequencies.
Mixers have a variety of uses, particularly in radio frequency (RF) electronic systems. For example, mixers are widely used to shift signals from one frequency range to another, a process known as heterodyning, for convenience in transmission or further signal processing. In addition, a mixer can be used as a phase detector, modulator, or demodulator, among other uses. In all cases, mixers exhibit a characteristic called linearity, which is a measure of how well the mixer converts its applied signals into the desired output while minimizing the amount of unwanted signals such as harmonics or higher order mixing products. Two well-known measures of linearity are the input-referenced second intercept point (IIP2) and the input-referenced third intercept point (IIP3). The harmonic frequencies and higher order sum and difference frequencies output by mixers are the principal reason that linearity is important in mixer design.
Mixers come in a variety of topologies for various applications. A particularly valuable topology is the double balanced mixer, used primarily to improve linearity beyond that of a single balanced mixer, especially for even-order mixing products. Typically, a double balanced mixer (DBM) has both its inputs applied to differential circuits, so that neither of the input signals and only the product signal appears at the output. The most common form of DBM is the diode DBM. In its simplest form, the diode DBM consists of two unbalanced-to-balanced transformers (also known as a “balun” transformer or simply “balun”) and a diode ring consisting of four diodes. A mixer has three ports: an RF port, a local oscillator (LO) port, and an intermediate frequency (IF) port. The RF port is where a high frequency signal is applied for down-conversion or where a high-frequency signal is output for up-conversion. The LO port is where the RF signal for the mixer is injected. The IF port is where a down-converted RF signal is passed, or where a signal to be up-converted to an RF signal is applied.
FIG. 1 is a schematic diagram of a conventional diode double balanced mixer 100. A first balun 102 is configured to receive an LO signal on the unbalanced side and generate two balanced LO output signals that are coupled to two corresponding opposing nodes of a diode ring 104. The ring diode 104 often comprises Schottky diodes, but may comprise other switching elements such as conventional diodes or LO-gated FETs. A second balun 106 is configured to pass an RF signal at a corresponding port on the unbalanced side of the second balun 106. The two legs of the balanced side of the second balun 106 couple the RF signal to the remaining two opposing nodes of the diode ring 104. The IF signal port is coupled to the center tap of the second balun 106.
A problem with conventional diode DBMs is that their conversion gain (CG) rapidly decreases as LO power decreases, due to the turn-on voltage of the diodes. For example, a typical silicon diode has a 0.7V turn-on voltage. Conversion gain (or loss, when the gain is negative) is the ratio of the desired IF output (voltage or power) to the RF input signal value (voltage or power). A typical diode DBM has a CG of approximately −6 dB. The diode turn-on voltage also makes it impractical to use a diode ring with more than one diode per branch for linearity improvement, due to the large LO drive signal that would be required.
Another problem with conventional diode DBMs is that their third-order input-referenced intercept point (IIP3) value increases at a slow rate versus LO power. The IIP3 measure is the RF input power at which the output power levels of unwanted intermodulation products and a desired IF output would be equal, and is a widely used measure of linearity in RF systems.
Accordingly, there is a need for a double balanced mixer that exhibits good conversion gain and IIP3 values, and which provide improved linearity and wide bandwidth. The present invention meets these needs.